...what's the best way to reply the student?
"
I don't know."
If it is important, look it up together before the flight (all students carry iPhones and/or iPads with permanent internet access with them novadays, so this should not be a problem). If it is not important, look it up together after the flight. But it is really important to clarify the open question
together, because a) you will learn something too, b) you will show the student how and where to gather relevant information and c) it's an excercise in teamwork that is an essential basis for all kinds aviation.
Happy landings,
max